Relative time allows geologists to determine the order of geologic
events. It characterizes geologic time as a chronology, with no absolute
dates. Rock formations are simply classified as older or younger relative
to other formations.

Geologists determine a rock's
origin by observing its physical attributes. Certain characteristics,
such as a rock's chemical composition and the way its constituent minerals
are arranged, give clues about the environment in which the rock was
formed. For example, was the rock formed during a volcanic eruption
or from materials deposited in a riverbed? Moreover, by looking at an
outcrop, it is possible to determine the order of events that resulted
in the rocks (or sediments)
that it contains. Such determinations are made using six
main principles that are central to relative geologic time.